The present invention relates to acoustic or ultrasonic transducers, and more particularly to a transducer assembly designed for flowmeter applications. Such electroacoustic transducers have found wide application in the non-destructive testing of material as well as in flowmeter applications.
For flowmeter applications, a pair of such transducers are typically used with each transducer alternately serving as a transmitter and receiver. It is also the common practice to shock excite the transmitting transducer with a voltage pulse and to detect the first or second half cycle of the received ultrasonic wavefront, such a technique being known as leading edge detection. For such a mode of operation, the received wavefront should be sharp and clean, and the ringing of the receiving transducer following its use as a transmitter should decay quickly, so that subsequent arriving wavefronts can be easily detected. It is thus desirable to minimize the mechanical Q of both the plezoelectric transducer element and the acoustic window. The transducer, and in particular its acoustic window, should be structured to detect the arriving ultrasonic signal while reflecting as little energy as possible back into the medium. This is to avoid measurement errors which may arise from sensing of reflected signals, as well as to obtain maximum sensitivity.
When the transducer device is used in a high temperature corrosive fluid medium under high pressure, it is desirable that the acoustic window be made of a high temperature, high strength material which is chemically resistant to attack by the medium.
It is desirable that the piezoelectric element be replaceable in the transducer assembly and that such replacement be effected while the assembly is in position in the fluid medium. It has been the practice to adhesively bond a thin disk of piezoelectric material to an acoustic window in most ultrasonic flowmeter transducer constructions. The adhesive bonding holds the piezoelectric element in place and provides relatively good acoustic coupling between the window and the piezoelectric element, however, the adhesive bond prevents ready replacement of the piezoelectric element. Also, organic adhesive bonds may degrade at elevated temperatures, and the technique of applying metallic adhesive bonds, such as brazing or soldering, may degrade the sensitivity of piezoelectric elements by depoling.